CHP can inspect any vehicle they like, but they usually leave the commercial vehicles to a handful of officers in each office who specialize in commercial vehicle enforcement. Easy to recognize, they're the ones who usually drive the (almost) unmarked stealth cop cars.
But any CHP traffic officer can do certain compliance checks almost anytime, especially if they witness a moving violation or compliance issue. If they get too many complaints because they're harassing people, they'll get slapped down by the Sgt for causing trouble.